Conventionally, as a gas detection apparatus such as above, for example, there is the gas monitoring apparatus described in PTL 1 indicated below.
In the gas monitoring apparatus described in PTL 1, an infrared monitoring camera takes an image of infrared rays radiated or infrared rays reflected from the monitoring area, and includes a tunable filter that can change a transmission spectrum thereof. An image processing apparatus calculates a difference between an image taken with the tunable filter adjusted so as to have a transmission peak at one of absorption bands of a particular detection target gas (hereinafter referred to as a “first image”) and an image taken with the tunable filter adjusted so as to have a transmittance peak near the absorption band thereof (hereinafter referred to as a “second image”). Subsequently, the image processing apparatus generates a differential image including, as an image of the gas, a pixel area in which infrared rays radiated or infrared rays reflected from the background of the monitoring area have been absorbed by the detection target gas.